path: root/e3/60d9e5fb58284db62bc44855528dad7c3be1cf

Return-Path: <riccardo.casatta@gmail.com>
Received: from smtp1.linuxfoundation.org (smtp1.linux-foundation.org
	[172.17.192.35])
	by mail.linuxfoundation.org (Postfix) with ESMTPS id 2EACA9C0
	for <bitcoin-dev@lists.linuxfoundation.org>;
	Thu, 29 Mar 2018 08:17:37 +0000 (UTC)
X-Greylist: whitelisted by SQLgrey-1.7.6
Received: from mail-it0-f46.google.com (mail-it0-f46.google.com
	[209.85.214.46])
	by smtp1.linuxfoundation.org (Postfix) with ESMTPS id 6FA4A5E1
	for <bitcoin-dev@lists.linuxfoundation.org>;
	Thu, 29 Mar 2018 08:17:34 +0000 (UTC)
Received: by mail-it0-f46.google.com with SMTP id m134-v6so7141429itb.3
	for <bitcoin-dev@lists.linuxfoundation.org>;
	Thu, 29 Mar 2018 01:17:34 -0700 (PDT)
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025;
	h=mime-version:in-reply-to:references:from:date:message-id:subject:to; 
	bh=qkXkRdJDmBMQHyVrEEaXT0qHJMj+ewOPZ3NBZayvpOg=;
	b=Zy/B/hEr0wL6vKslBd3xFeA+CFPordpL9yK2VkdMRQdI/YkNiywu99R6YVUqklTG9I
	gp7ccblUpwKbqKssnV5xLwJpyKL3HcA0FZsUB14ZpOGeOq3dknTfCLXlC0nAufvUh5ej
	/wsgTjgzw8L79FVNVIt2z6VTwPTlybLIXtVuyn+UepDZ4O+t4+EHwIcdviVcu+Xv+s+K
	tPMSvA5rHHJSXMW8iynhGmJ4xz1Ejcddh0C/d0GFUoYGunzNqQ9MYea1jln7hL5eQK6n
	I1bprefzmaoOBYLGVCxNqiCMSYtvdo6huBn9WUUbfohBk8xtPln8f6KRd3L3LV6PVEJS
	e7+Q==
X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;
	d=1e100.net; s=20161025;
	h=x-gm-message-state:mime-version:in-reply-to:references:from:date
	:message-id:subject:to;
	bh=qkXkRdJDmBMQHyVrEEaXT0qHJMj+ewOPZ3NBZayvpOg=;
	b=HH7WAfJiGsML6PFNxNzZDOQ8lYiltTt9TQ9Vs1x8xNi05bMBSkE0WCS9j5I1/WkmZM
	kRQwxIywZwpMmpRC5en8O6Ggh3RF2YwE4M7U3YxLf+nB4gK43KmM4kcUCyLV32g78Pfy
	46tXqrsZxxBnzCfve3rIZyC7M47M1H9vaNPeJ3r1MsXaInq922sJaY6GWX23rYhTRgu0
	epvNbS4x/MfzqdgP020JuvhV7d19rFd/R0cR9kvOqXowIPBdK8TCI75J1Vfqf+pYYsxE
	3fRSiruzX5nzUbPNtKJrJK39E/7Qoca9gjJ1urd91JFIEbkJQ02p1ovwhIzWXOHM6sQF
	nTBg==
X-Gm-Message-State: AElRT7EaVOm4MFCdqmiD/rNtIX8K5jQAJ1COW7D4hXs05sLlJQemcnlX
	YNTxGdK6VLET8AxrGEY9FApCkFTw76XpDd/i5J8=
X-Google-Smtp-Source: AIpwx4/ROxNGqyqSVbS6gUmERLJ+RwSRHzMJh75Jg0/bVZfA+Tma+GWVPjTXzKnIg7dobWiZklvpXA5f59LfbOAnvO8=
X-Received: by 2002:a24:cac6:: with SMTP id
	k189-v6mr2482451itg.57.1522311453382; 
	Thu, 29 Mar 2018 01:17:33 -0700 (PDT)
MIME-Version: 1.0
Received: by 10.2.46.32 with HTTP; Thu, 29 Mar 2018 01:17:12 -0700 (PDT)
In-Reply-To: <CADZtCSg7+x-sg-ysgacXobRexOVwT+k9fr6a9S-6xU2w8f8m3A@mail.gmail.com>
References: <CADZtCSg7+x-sg-ysgacXobRexOVwT+k9fr6a9S-6xU2w8f8m3A@mail.gmail.com>
From: Riccardo Casatta <riccardo.casatta@gmail.com>
Date: Thu, 29 Mar 2018 10:17:12 +0200
Message-ID: <CADabwBAjTRdVqsL+V=YdQ+kr8+LtSPOeSXUJOzKoPNdKEbAOWQ@mail.gmail.com>
To: Jim Posen <jim.posen@gmail.com>, 
	Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
Content-Type: multipart/alternative; boundary="0000000000006aa04f056888c1c9"
X-Spam-Status: No, score=-2.0 required=5.0 tests=BAYES_00,DKIM_SIGNED,
	DKIM_VALID, DKIM_VALID_AU, FREEMAIL_FROM, HTML_MESSAGE,
	RCVD_IN_DNSWL_NONE autolearn=ham version=3.3.1
X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on
	smtp1.linux-foundation.org
X-Mailman-Approved-At: Thu, 29 Mar 2018 18:16:14 +0000
Subject: Re: [bitcoin-dev] Optimized Header Sync
X-BeenThere: bitcoin-dev@lists.linuxfoundation.org
X-Mailman-Version: 2.1.12
Precedence: list
List-Id: Bitcoin Protocol Discussion <bitcoin-dev.lists.linuxfoundation.org>
List-Unsubscribe: <https://lists.linuxfoundation.org/mailman/options/bitcoin-dev>,
	<mailto:bitcoin-dev-request@lists.linuxfoundation.org?subject=unsubscribe>
List-Archive: <http://lists.linuxfoundation.org/pipermail/bitcoin-dev/>
List-Post: <mailto:bitcoin-dev@lists.linuxfoundation.org>
List-Help: <mailto:bitcoin-dev-request@lists.linuxfoundation.org?subject=help>
List-Subscribe: <https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev>,
	<mailto:bitcoin-dev-request@lists.linuxfoundation.org?subject=subscribe>
X-List-Received-Date: Thu, 29 Mar 2018 08:17:37 -0000

--0000000000006aa04f056888c1c9
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable

Hi Jim,

| <code>version[i] =3D version[i - ((bit[3] << 2) + (bit[4] << 1) +
bit[5])]</code>


Thought this wasn't effective in case overt asic boost get widely adopted,
but then I understood that at the moment only two bits of version get
scrambled by that technique so this looks fine, maybe add a comment about
this so the reader doesn't get the same initial doubt I got.

...downloading evenly spaced checkpoints throughout history (say every
> 1,000th) from all peers first...


My feeling is that encoding of the headers and checkpoints/parallel
download are separate subjects for two BIPS.
About the checkpoints I don't grasp why they are useful since an attacker
could lie about them but maybe I am missing something...

To take advantage of these possible savings, this document defines a
> variable-sized ''compressed encoding'' of block headers that occur in a
> range. Note that no savings are possible when serializing a single header=
;
> it should only be used for vectors of sequential headers. The full header=
s
> are reconstructed using data from previous headers in the range. The
> serialization begins with an ''encoding indicator'', which is a bitfield
> specifying how each field is serialized. The bits of the indicator have t=
he
> following semantics:


Bitfield allows great savings, however the encoding depends on the headers
height a client ask for, this cause a little computational burden on the
node and the undesirable side effect of difficult caching. Variable length
encoding cause caching difficulties too...
A simpler approach could be to encode the headers in groups of 2016 headers
(the difficulty period) where the first header is complete and the others
2015 are missing the previous hash and the difficulty, this achieve
comparable savings ~45%, allows better caching and has fixed length
encoding. This could be useful for the node by caching headers on a single
file on disk and simply stream out the relative range when requested or to
serve the same encoded headers format in other context like http,
leveraging http caching infrastructure.



2018-03-28 1:31 GMT+02:00 Jim Posen via bitcoin-dev <
bitcoin-dev@lists.linuxfoundation.org>:

> Based on some ideas that were thrown around in this thread (https://lists=
.
> linuxfoundation.org/pipermail/bitcoin-dev/2017-December/015385.html), I
> have been working on a P2P extension that will allow faster header sync
> mechanisms. The one-sentence summary is that by encoding headers more
> efficiently (eg. omitting prev_hash) and downloading evenly spaced
> checkpoints throughout history (say every 1,000th) from all peers first, =
we
> could speed up header sync, which would be a huge improvement for light
> clients. Here is a draft of the BIP: https://github.com/jimpo/
> bips/blob/headers-sync/headersv2.mediawiki. The full text is below as
> well.
>
> I'd love to hear any feedback people have.
>
> ----------------------------------------------------------
>
> =3D=3D Abstract =3D=3D
>
> This BIP describes a P2P network extension enabling faster, more reliable=
 methods for syncing the block header chain. New P2P messages are proposed =
as more efficient replacements for <code>getheaders</code> and <code>header=
s</code> during initial block download. The proposed header download protoc=
ol reduces bandwidth usage by ~40%-50% and supports downloading headers ran=
ges from multiple peers in parallel, which is not possible with the current=
 mechanism. This also enables sync strategies with better resistance to den=
ial-of-service attacks.
>
> =3D=3D Motivation =3D=3D
>
> Since 2015, optimized Bitcoin clients fetch all block headers before bloc=
ks themselves in order to avoid downloading ones that are not part of the m=
ost work chain. The protocol currently in use for fetching headers leaves r=
oom for further optimization, specifically by compressing header data and d=
ownloading more headers simulaneously<ref>https://lists.linuxfoundation.org=
/pipermail/bitcoin-dev/2017-December/015385.html</ref>. Any savings here sh=
ould have a large impact given that both full nodes and light clients must =
sync the header chain as a first step, and that the time to validate and in=
dex the headers is negligible compared to the time spent downloading them f=
rom the network. Furthermore, some current implementations of headers synci=
ng rely on preconfigured checkpoints to discourage attackers attempting to =
fill up a victim's disk space with low-work headers. The proposed messages =
enable sync strategies that are resilient against these types of attacks. T=
he P2P messages are designed to be flexible, supporting multiple header syn=
c strategies and leaving room for future innovations, while also compact.
>
> =3D=3D Definitions =3D=3D
>
> ''double-SHA256'' is a hash algorithm defined by two invocations of SHA-2=
56: <code>double-SHA256(x) =3D SHA256(SHA256(x))</code>.
>
> =3D=3D Specification =3D=3D
>
> The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOU=
LD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document ar=
e to be interpreted as described in RFC 2119.
>
> =3D=3D=3D New Structures =3D=3D=3D
>
> =3D=3D=3D=3D Compressed Headers =3D=3D=3D=3D
>
> Bitcoin headers are serialized by default in 80 bytes as follows:
>
> {| class=3D"wikitable"
> ! Field Name
> ! Data Type
> ! Byte Size
> ! Description
> |-
> | version
> | int32_t
> | 4
> | Block version information
> |-
> | prev_block
> | uint256
> | 32
> | The hash of the previous block
> |-
> | merkle_root
> | uint256
> | 32
> | The root hash of the transaction Merkle tree
> |-
> | timestamp
> | uint32_t
> | 4
> | A Unix timestamp of the block creation time, as reported by the miner
> |-
> | bits
> | uint32_t
> | 4
> | The calculated difficulty target for this block
> |-
> | nonce
> | uint32_t
> | 4
> | A nonce that is set such that the header's hash matches the difficulty =
target
> |}
>
> When deserializing a correctly-formed sequence of block headers encoded i=
n this way, it can be noted that:
>
> * The prev_block field should always match the double-SHA256 hash of the =
previous header, making it redundant
> * According to Bitcoin consensus rules, the bits field only changes every=
 2016 blocks
> * The version often matches that of a recent ancestor block
> * The timestamp is often a small delta from the preceding header's timest=
amp
>
> To take advantage of these possible savings, this document defines a vari=
able-sized ''compressed encoding'' of block headers that occur in a range. =
Note that no savings are possible when serializing a single header; it shou=
ld only be used for vectors of sequential headers. The full headers are rec=
onstructed using data from previous headers in the range. The serialization=
 begins with an ''encoding indicator'', which is a bitfield specifying how =
each field is serialized. The bits of the indicator have the following sema=
ntics:
>
> {| class=3D"wikitable"
> ! Bit Index
> ! Reconstruction
> ! Description
> |-
> | 0
> | <code>prev_block[i] =3D DSHA256(header[i-1])</code>
> | The prev_block field is ommitted and assigned to the double-SHA256 hash=
 of the previous uncompressed header.
> |-
> | 1
> | <code>nbits[i] =3D nbits[i-1]</code>
> | The nbits field is omitted and matches that of the previous header.
> |-
> | 2
> | <code>timestamp[i] =3D timestamp[i-1] + value</code>
> | The timestamp is replaced by a 2-byte signed short int, representing an=
 offset from the previous block's timestamp
> |-
> | 3
> |
> | Interpreted along with bits 4 & 5.
> |-
> | 4
> |
> | Interpreted along with bits 3 & 5.
> |-
> | 5
> | <code>version[i] =3D version[i - ((bit[3] << 2) + (bit[4] << 1) + bit[5=
])]</code>
> | Bits 3, 4, and 5 are first interpreted as a 3-bit offset, with bit inde=
x 3 as the most significant and bit index 5 as the least significant. If th=
e offset is non-zero, the version field is omitted and assigned to the vers=
ion of the block at the offset number of blocks prior.
> |-
> | 6
> |
> | Reserved.
> |-
> | 7
> |
> | Reserved. May be used in a future encoding version to signal another in=
dicator byte.
> |}
>
> The compressed header format is versioned by a 256-bit unsigned integer. =
This document defines version 0.
>
> =3D=3D=3D=3D VarInt =3D=3D=3D=3D
>
> ''VarInt'' is a variable-length unsigned integer encoding that supports a=
 greater range of numbers than the standard ''CompactSize''. This encoding =
was introduced at the database layer in Bitcoin Core<ref>https://github.com=
/bitcoin/bitcoin/commit/4d6144f97faf9d2a6c89f41d7d2360f21f0b71e2</ref> in 2=
012, but is new to the Bitcoin P2P layer.
>
> This definition is per the code comments in Bitcoin Core written by Piete=
r Wuille:
>
> <pre>
> Variable-length integers: bytes are a MSB base-128 encoding of the number=
.
> The high bit in each byte signifies whether another digit follows. To mak=
e
> the encoding is one-to-one, one is subtracted from all but the last digit=
.
> Thus, the byte sequence a[] with length len, where all but the last byte
> has bit 128 set, encodes the number:
>
>   (a[len-1] & 0x7F) + sum(i=3D1..len-1, 128^i*((a[len-i-1] & 0x7F)+1))
>
> Properties:
> * Very small (0-127: 1 byte, 128-16511: 2 bytes, 16512-2113663: 3 bytes)
> * Every integer has exactly one encoding
> * Encoding does not depend on size of original integer type
> * No redundancy: every (infinite) byte sequence corresponds to a list
>   of encoded integers.
>
> 0:         [0x00]  256:        [0x81 0x00]
> 1:         [0x01]  16383:      [0xFE 0x7F]
> 127:       [0x7F]  16384:      [0xFF 0x00]
> 128:  [0x80 0x00]  16511: [0x80 0xFF 0x7F]
> 255:  [0x80 0x7F]  65535: [0x82 0xFD 0x7F]
> 2^32:           [0x8E 0xFE 0xFE 0xFF 0x00]
> </pre>
>
> =3D=3D=3D=3D Checkpoints =3D=3D=3D=3D
>
> A ''checkpoint'' is defined for a block as a tuple of its hash and the ch=
ain work:
>
> {| class=3D"wikitable"
> ! Field Name
> ! Data Type
> ! Byte Size
> ! Description
> |-
> | block_hash
> | uint256
> | 32
> | The hash of the block
> |-
> | chain_work
> | VarInt
> | Variable(1-20)
> | A delta between the total work in the chain at the checkpoint block and=
 a previous checkpoint, determined by context
> |}
>
> =3D=3D=3D Service Bit =3D=3D=3D
>
> This BIP allocates a new service bit:
>
> {| class=3D"wikitable"
> |-
> | NODE_HEADERS_V2
> | <code>1 << ?</code>
> | If enabled, the node MUST respond to <code>getcheckpts</code> and <code=
>getheaders2</code> queries
> |}
>
> =3D=3D=3D New Messages =3D=3D=3D
>
> =3D=3D=3D=3D getcheckpts =3D=3D=3D=3D
> <code>getcheckpts</code> is used to request block headers at a specified =
distance from each other which serve as checkpoints during parallel header =
download. The message contains the following fields:
>
> {| class=3D"wikitable"
> ! Field Name
> ! Data Type
> ! Byte Size
> ! Description
> |-
> | block_locator
> | uint256[]
> | Variable
> | A vector of block hashes in descending order by height used to identify=
 the header chain of the requesting node
> |-
> | interval
> | uint32_t
> | 4
> | The distance in block height between requested block hashes
> |}
>
> # Nodes SHOULD NOT send <code>getcheckpts</code> unless the peer has set =
the <code>NODE_HEADERS_V2</code> service bit
> # The hashes in <code>block_locator</code> MUST be in descending order by=
 block height
> # The block locator SHOULD be generated as it is in <code>getheaders</cod=
e> requests
> # The receiving node MUST respond to valid requests with a <code>checkpts=
</code> response where the interval is the same as in the request and the f=
irst checkpoint hash matches the first common block hash in the block locat=
or
>
> =3D=3D=3D=3D checkpts =3D=3D=3D=3D
> <code>checkpts</code> is sent in response to <code>getcheckpts</code>, li=
sting block hashes at the specified interval. The message contains the foll=
owing fields:
>
> {| class=3D"wikitable"
> ! Field Name
> ! Data Type
> ! Byte Size
> ! Description
> |-
> | start_height
> | uint32_t
> | 4
> | The height of the first block in the active chain matching the request'=
s block locator
> |-
> | end_height
> | uint32_t
> | 4
> | The height of the last block in the active chain
> |-
> | start_checkpoint
> | Checkpoint
> | 48
> | The checkpoint structure for the block in the active chain at height st=
art_height
> |-
> | end_checkpoint
> | Checkpoint
> | 48
> | The checkpoint structure for the block in the active chain at height en=
d_height
> |-
> | interval
> | uint32_t
> | 4
> | The distance in block height between checkpoints
> |-
> | checkpoints_length
> | CompactSize
> | Variable(1-5)
> | The number of checkpoints to follow
> |-
> | checkpoints
> | Checkpoint[]
> | checkoints_length * Variable(33-52)
> | The checkpoints as specified below
> |}
>
> # The interval SHOULD match the field in the <code>getcheckpts</code> req=
uest
> # The start_checkpoint SHOULD correspond to the first block hash in the l=
ocator from the <code>getcheckpts</code> request that is part of the active=
 chain
> # The end_checkpoint SHOULD correspond to the tip of the node's active ch=
ain
> # The start_height MOST be set to the block height of the start_checkpoin=
t
> # The end_height MOST be set to the block height of the end_checkpoint
> # If the interval is zero, the checkpoints vector MUST be empty
> # If the interval is non-zero, checkpoints MUST correspond to blocks on t=
he active chain between the start_checkpoint and the end_checkpoint (exclus=
ive), where the difference in block height between each entry and the previ=
ous one is equal to the interval
> # The checkpoints_length MUST be less than or equal to 2,000
> # The node SHOULD include as many checkpoints on its active chain as are =
available, up to the limit of 2,000
> # The chain_work field in the first checkpoint MUST be the total work in =
the chain ending at that block
> # The chain_work field in each subsequent checkpoint MUST be the differen=
ce in chain work between that block and the previous checkpoint
> # The chain_work field in each checkpoint MUST be a properly-encoded VarI=
nt, not exceeding 20 bytes
>
> =3D=3D=3D=3D getheaders2 =3D=3D=3D=3D
> <code>getheaders2</code> is used to request compressed headers for a rang=
e of blocks. The message contains the following fields:
>
> {| class=3D"wikitable"
> ! Field Name
> ! Data Type
> ! Byte Size
> ! Description
> |-
> | max_version
> | uint8_t
> | 1
> | The maximum supported encoding version of the headers
> |-
> | flags
> | uint8_t
> | 1
> | A bitfield of message encoding flags
> |-
> | start_height
> | uint32_t
> | 4
> | The height of the first block header in the requested range
> |-
> | end_hash
> | uint256
> | 32
> | The hash of the last block header in the requested range
> |}
>
> # Nodes SHOULD NOT send <code>getheaders2</code> unless the peer has set =
the <code>NODE_HEADERS_V2</code> service bit
> # The height of the block with hash end_hash MUST be greater than or equa=
l to start_height, and the difference MUST be strictly less than 3,000
> # The end_hash SHOULD match one in a previously received <code>checkpts</=
code> message, otherwise the receiving node MAY disconnect
> # The 0th bit (least significant order) of the flags field MAY be set to =
request the coinbase transaction and merkle branch for the block at height =
start_height
>
> =3D=3D=3D=3D headers2 =3D=3D=3D=3D
> <code>headers2</code> is sent in response to <code>getheaders2</code>, li=
sting the compressed headers in the requested range. The message contains t=
he following fields:
>
> {| class=3D"wikitable"
> ! Field Name
> ! Data Type
> ! Byte Size
> ! Description
> |-
> | version
> | uint8_t
> | 1
> | The encoding version of the headers
> |-
> | flags
> | uint8_t
> | 1
> | A bitfield of message encoding flags
> |-
> | start_height
> | uint32_t
> | 4
> | The height of the first block header returned
> |-
> | headers_length
> | CompactSize
> | 1-3
> | The number of block headers to follow
> |-
> | headers
> | CompressedHeader[]
> | Variable
> | The compressed block headers
> |-
> | start_block_coinbase_tx
> | CTransaction
> | Variable
> | The coinbase transaction in the block at start_height
> |-
> | start_block_coinbase_branch
> | uint256[]
> | Variable
> | A merkle branch linking the coinbase transaction in the block at start_=
height to its header
> |}
>
> # The version MUST be less than or equal to the max_version field of the =
<code>getheaders2</code> request
> # Any bits set in the flags field of the <code>getheaders2</code> request=
 MAY be set in the response field
> # Any bits not set in the flags field of the <code>getheaders2</code> req=
uest MUST NOT be set in the response field
> # The first header MUST be encoded with a 0-byte indicator (ie. the heade=
r is uncompressed)
> # start_height MUST be set to the block height of the first header
> # The hash of the last block SHOULD equal the end_hash of the <code>gethe=
aders2</code> request, ''even if the block is no longer part of the active =
chain''
> # The length of the headers vector MUST be less than or equal to 3,000
> # The headers MUST be sequential in order of height, with each header a s=
uccessor of the previous one
> # Each header SHOULD be optimally compressed
> # The start_block_coinbase_tx should be the serialized coinbase transacti=
on in the block corresponding to the first header
> # The start_block_coinbase_branch should be a vector of right-hand-side h=
ashes in the merkle branch linking the coinbase transaction to the first he=
ader, in order from bottom of the tree to top
> # If the 0th bit (least significant order) of the flags field is unset, t=
he start_block_coinbase_tx and start_block_coinbase_branch fields MUST be o=
mitted
>
> =3D=3D=3D Sync Strategies =3D=3D=3D
>
> The general header sync protocol for clients now is to first request chec=
kpoints from all peers with <code>getcheckpts</code>, then decide which pee=
rs to fetch ranges of headers from and download them with <code>getheaders2=
</code>.
>
> =3D=3D=3D=3D Forward Sequential Syncing =3D=3D=3D=3D
>
> Similar to the current sync protocol, a client may choose one peer to dow=
nload headers from, then fetch them in forward sequential order. Once this =
peer is out of headers, the client performs the same routine with any peers=
 offering more headers.
>
> With this strategy, the client is able to fully validate the block header=
s in order and abort if the peer serves an invalid one. On the other hand, =
the peer may be able to serve a longer, lower-work chain than the global ac=
tive chain, wasting the client's time, memory, and storage space.
>
> =3D=3D=3D=3D Parallel Header Download =3D=3D=3D=3D
>
> In order to increase the throughput of header downloads, a node may downl=
oad multiple header ranges in parallel from all peers serving the same chec=
kpoints, then validate them in sequential order.
>
> =3D=3D=3D=3D Random Sampling Proof-of-Work  =3D=3D=3D=3D
>
> Similar the FlyClient<ref>https://www.youtube.com/watch?time_continue=3D8=
400&v=3DBPNs9EVxWrA</ref> header download protocol, clients can select the =
peer claiming the greatest total work chain and use random sampling to effi=
ciently determine if the peer is likely to be reporting its chain work hone=
stly.
>
> The client treats the checkpoint message as a commitment to chain work of=
 intermediate ranges of headers, the client then randomly samples ranges of=
 headers weighted by total work to determine whether the total chain work i=
s valid before downloading all headers. To defend against malicious peers a=
ttempting to reuse earlier headers later in the chain to fake greater total=
 work, the client should check the block height in the coinbase transaction=
 for all headers after the BIP 34 activation height. If the peer is found t=
o be dishonest, they can be banned before the client downloads too many hea=
ders, otherwise the client chooses this as the primary sync peer for forwar=
d sequential sync or parallel download.
>
> =3D=3D Rationale =3D=3D
>
> * '''Why include the coinbase transaction in the headers messages?''' The=
 primary reason is that after BIP 34<ref>https://github.com/bitcoin/bips/bl=
ob/master/bip-0034.mediawiki</ref> activation at block height 227,835, coin=
base transactions constitute cryptographic commitments to a block's height =
in the chain, which mitigates certain attacks during header sync. Furthermo=
re, the <code>getheaders2</code> message can be used as a simple way of req=
uesting a coinbase transaction for a single header, which may be independen=
tly useful.
>
> * '''Why not omit nBits entirely?''' The compression is designed to permi=
t full decompression of all headers in a <code>headers2</code> message ''wi=
thout'' requiring any other chain context. This is desirable so that proofs=
 of work may be validated for arbitrary header ranges. While nBits can be c=
omputed knowing previous headers, this requires block headers that may not =
be sent in the same message.
>
> =3D=3D Compatibility =3D=3D
>
> This is backwards compatible, as it defines new P2P messages which are av=
ailable if a service bit is signaled. There are no changes to consensus rul=
es.
>
> =3D=3D Acknowledgements =3D=3D
>
> Thanks to Gregory Maxwell for suggestions on the compressed header encodi=
ng and the DOS-resistant sync strategies. Thanks to Suhas Daftuar for helpf=
ul discussions.
>
> Credit for the VarInt encoding goes to Pieter Wuille.
>
>
> _______________________________________________
> bitcoin-dev mailing list
> bitcoin-dev@lists.linuxfoundation.org
> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>
>


--=20
Riccardo Casatta - @RCasatta <https://twitter.com/RCasatta>

--0000000000006aa04f056888c1c9
Content-Type: text/html; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable

<div dir=3D"ltr">Hi Jim,<div><br></div><div><pre style=3D"font-style:normal=
;font-variant-ligatures:normal;font-variant-caps:normal;font-weight:400;let=
ter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;wor=
d-spacing:0px;background-color:rgb(255,255,255);text-decoration-style:initi=
al;text-decoration-color:initial;color:rgb(0,0,0);word-wrap:break-word;whit=
e-space:pre-wrap">| &lt;code&gt;version[i] =3D version[i - ((bit[3] &lt;&lt=
; 2) + (bit[4] &lt;&lt; 1) + bit[5])]&lt;/code&gt;</pre><div><br></div><div=
>Thought this wasn&#39;t effective in case overt asic boost get widely adop=
ted, but then I understood that at the moment only two bits of version get =
scrambled by that technique so this looks fine, maybe add a comment about t=
his so the reader doesn&#39;t get the same initial doubt I got.</div><div><=
br></div><blockquote class=3D"gmail_quote" style=3D"margin:0px 0px 0px 0.8e=
x;border-left:1px solid rgb(204,204,204);padding-left:1ex"><span style=3D"c=
olor:rgb(34,34,34);font-family:arial,sans-serif;font-size:small;font-style:=
normal;font-variant-ligatures:normal;font-variant-caps:normal;font-weight:4=
00;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:no=
ne;white-space:normal;word-spacing:0px;background-color:rgb(255,255,255);te=
xt-decoration-style:initial;text-decoration-color:initial;float:none;displa=
y:inline">...downloading evenly spaced checkpoints throughout history (say =
every 1,000th) from all peers first...</span></blockquote><div><br></div><d=
iv>My feeling is that encoding of the headers and checkpoints/parallel down=
load are separate subjects for two BIPS.</div><div>About the checkpoints I =
don&#39;t grasp why they are useful since an attacker could lie about them =
but maybe I am missing something...</div><div><br></div><blockquote class=
=3D"gmail_quote" style=3D"margin:0px 0px 0px 0.8ex;border-left:1px solid rg=
b(204,204,204);padding-left:1ex">To take advantage of these possible saving=
s, this document defines a variable-sized &#39;&#39;compressed encoding&#39=
;&#39; of block headers that occur in a range. Note that no savings are pos=
sible when serializing a single header; it should only be used for vectors =
of sequential headers. The full headers are reconstructed using data from p=
revious headers in the range. The serialization begins with an &#39;&#39;en=
coding indicator&#39;&#39;, which is a bitfield specifying how each field i=
s serialized. The bits of the indicator have the following semantics:</bloc=
kquote><div><br></div><div>Bitfield allows great savings, however the encod=
ing depends on the headers height a client ask for, this cause a little com=
putational burden on the node and the undesirable side effect of difficult =
caching. Variable length encoding cause caching difficulties too...</div><d=
iv>A simpler approach could be to encode the headers in groups of 2016 head=
ers (the difficulty period) where the first header is complete and the othe=
rs 2015 are missing the previous hash and the difficulty, this achieve comp=
arable savings ~45%, allows better caching and has fixed length encoding. T=
his could be useful for the node by caching headers on a single file on dis=
k and simply stream out the relative range when requested or to serve the s=
ame encoded headers=C2=A0<span style=3D"color:rgb(34,34,34);font-family:ari=
al,sans-serif;font-size:small;font-style:normal;font-variant-ligatures:norm=
al;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-alig=
n:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing=
:0px;background-color:rgb(255,255,255);text-decoration-style:initial;text-d=
ecoration-color:initial;float:none;display:inline">format</span> in other c=
ontext like http, leveraging http caching infrastructure.</div><div><br></d=
iv><br><div class=3D"gmail_extra"><br><div class=3D"gmail_quote">2018-03-28=
 1:31 GMT+02:00 Jim Posen via bitcoin-dev <span dir=3D"ltr">&lt;<a href=3D"=
mailto:bitcoin-dev@lists.linuxfoundation.org" target=3D"_blank">bitcoin-dev=
@lists.linuxfoundation.org</a>&gt;</span>:<br><blockquote class=3D"gmail_qu=
ote" style=3D"margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex=
"><div dir=3D"ltr">Based on some ideas that were thrown around in this thre=
ad (<a href=3D"https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2017=
-December/015385.html" target=3D"_blank">https://lists.<wbr>linuxfoundation=
.org/pipermail/<wbr>bitcoin-dev/2017-December/<wbr>015385.html</a>), I have=
 been working on a P2P extension that will allow faster header sync mechani=
sms. The one-sentence summary is that by encoding headers more efficiently =
(eg. omitting prev_hash) and downloading evenly spaced checkpoints througho=
ut history (say every 1,000th) from all peers first, we could speed up head=
er sync, which would be a huge improvement for light clients.=C2=A0Here is =
a draft of the BIP:=C2=A0<a href=3D"https://github.com/jimpo/bips/blob/head=
ers-sync/headersv2.mediawiki" target=3D"_blank">https://github.com/jimpo/<w=
br>bips/blob/headers-sync/<wbr>headersv2.mediawiki</a>. The full text is be=
low as well.<div><div><br></div><div>I&#39;d love to hear any feedback peop=
le have.</div></div><div><br></div><div>------------------------------<wbr>=
----------------------------</div><div><br></div><div><pre style=3D"color:r=
gb(0,0,0);font-style:normal;font-variant-ligatures:normal;font-variant-caps=
:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:=
0px;text-transform:none;word-spacing:0px;text-decoration-style:initial;text=
-decoration-color:initial;word-wrap:break-word;white-space:pre-wrap">=3D=3D=
 Abstract =3D=3D

This BIP describes a P2P network extension enabling faster, more reliable m=
ethods for syncing the block header chain. New P2P messages are proposed as=
 more efficient replacements for &lt;code&gt;getheaders&lt;/code&gt; and &l=
t;code&gt;headers&lt;/code&gt; during initial block download. The proposed =
header download protocol reduces bandwidth usage by ~40%-50% and supports d=
ownloading headers ranges from multiple peers in parallel, which is not pos=
sible with the current mechanism. This also enables sync strategies with be=
tter resistance to denial-of-service attacks.

=3D=3D Motivation =3D=3D

Since 2015, optimized Bitcoin clients fetch all block headers before blocks=
 themselves in order to avoid downloading ones that are not part of the mos=
t work chain. The protocol currently in use for fetching headers leaves roo=
m for further optimization, specifically by compressing header data and dow=
nloading more headers simulaneously&lt;ref&gt;<a href=3D"https://lists.linu=
xfoundation.org/pipermail/bitcoin-dev/2017-December/015385.html" target=3D"=
_blank">https://<wbr>lists.linuxfoundation.org/<wbr>pipermail/bitcoin-dev/2=
017-<wbr>December/015385.html</a>&lt;/ref&gt;. Any savings here should have=
 a large impact given that both full nodes and light clients must sync the =
header chain as a first step, and that the time to validate and index the h=
eaders is negligible compared to the time spent downloading them from the n=
etwork. Furthermore, some current implementations of headers syncing rely o=
n preconfigured checkpoints to discourage attackers attempting to fill up a=
 victim&#39;s disk space with low-work headers. The proposed messages enabl=
e sync strategies that are resilient against these types of attacks. The P2=
P messages are designed to be flexible, supporting multiple header sync str=
ategies and leaving room for future innovations, while also compact.

=3D=3D Definitions =3D=3D

&#39;&#39;double-SHA256&#39;&#39; is a hash algorithm defined by two invoca=
tions of SHA-256: &lt;code&gt;double-SHA256(x) =3D SHA256(SHA256(x))&lt;/co=
de&gt;.

=3D=3D Specification =3D=3D

The key words &quot;MUST&quot;, &quot;MUST NOT&quot;, &quot;REQUIRED&quot;,=
 &quot;SHALL&quot;, &quot;SHALL NOT&quot;, &quot;SHOULD&quot;, &quot;SHOULD=
 NOT&quot;, &quot;RECOMMENDED&quot;, &quot;MAY&quot;, and &quot;OPTIONAL&qu=
ot; in this document are to be interpreted as described in RFC 2119.

=3D=3D=3D New Structures =3D=3D=3D

=3D=3D=3D=3D Compressed Headers =3D=3D=3D=3D

Bitcoin headers are serialized by default in 80 bytes as follows:

{| class=3D&quot;wikitable&quot;
! Field Name
! Data Type
! Byte Size
! Description
|-
| version
| int32_t
| 4
| Block version information
|-
| prev_block
| uint256
| 32
| The hash of the previous block
|-
| merkle_root
| uint256
| 32
| The root hash of the transaction Merkle tree
|-
| timestamp
| uint32_t
| 4
| A Unix timestamp of the block creation time, as reported by the miner
|-
| bits
| uint32_t
| 4
| The calculated difficulty target for this block
|-
| nonce
| uint32_t
| 4
| A nonce that is set such that the header&#39;s hash matches the difficult=
y target
|}

When deserializing a correctly-formed sequence of block headers encoded in =
this way, it can be noted that:

* The prev_block field should always match the double-SHA256 hash of the pr=
evious header, making it redundant
* According to Bitcoin consensus rules, the bits field only changes every 2=
016 blocks
* The version often matches that of a recent ancestor block
* The timestamp is often a small delta from the preceding header&#39;s time=
stamp

To take advantage of these possible savings, this document defines a variab=
le-sized &#39;&#39;compressed encoding&#39;&#39; of block headers that occu=
r in a range. Note that no savings are possible when serializing a single h=
eader; it should only be used for vectors of sequential headers. The full h=
eaders are reconstructed using data from previous headers in the range. The=
 serialization begins with an &#39;&#39;encoding indicator&#39;&#39;, which=
 is a bitfield specifying how each field is serialized. The bits of the ind=
icator have the following semantics:

{| class=3D&quot;wikitable&quot;
! Bit Index
! Reconstruction
! Description
|-
| 0
| &lt;code&gt;prev_block[i] =3D DSHA256(header[i-1])&lt;/code&gt;
| The prev_block field is ommitted and assigned to the double-SHA256 hash o=
f the previous uncompressed header.
|-
| 1
| &lt;code&gt;nbits[i] =3D nbits[i-1]&lt;/code&gt;
| The nbits field is omitted and matches that of the previous header.
|-
| 2
| &lt;code&gt;timestamp[i] =3D timestamp[i-1] + value&lt;/code&gt;
| The timestamp is replaced by a 2-byte signed short int, representing an o=
ffset from the previous block&#39;s timestamp
|-
| 3
|
| Interpreted along with bits 4 &amp; 5.
|-
| 4
|
| Interpreted along with bits 3 &amp; 5.
|-
| 5
| &lt;code&gt;version[i] =3D version[i - ((bit[3] &lt;&lt; 2) + (bit[4] &lt=
;&lt; 1) + bit[5])]&lt;/code&gt;
| Bits 3, 4, and 5 are first interpreted as a 3-bit offset, with bit index =
3 as the most significant and bit index 5 as the least significant. If the =
offset is non-zero, the version field is omitted and assigned to the versio=
n of the block at the offset number of blocks prior.
|-
| 6
|
| Reserved.
|-
| 7
|
| Reserved. May be used in a future encoding version to signal another indi=
cator byte.
|}

The compressed header format is versioned by a 256-bit unsigned integer. Th=
is document defines version 0.

=3D=3D=3D=3D VarInt =3D=3D=3D=3D

&#39;&#39;VarInt&#39;&#39; is a variable-length unsigned integer encoding t=
hat supports a greater range of numbers than the standard &#39;&#39;Compact=
Size&#39;&#39;. This encoding was introduced at the database layer in Bitco=
in Core&lt;ref&gt;<a href=3D"https://github.com/bitcoin/bitcoin/commit/4d61=
44f97faf9d2a6c89f41d7d2360f21f0b71e2" target=3D"_blank">https://github.com/=
<wbr>bitcoin/bitcoin/commit/<wbr>4d6144f97faf9d2a6c89f41d7d2360<wbr>f21f0b7=
1e2</a>&lt;/ref&gt; in 2012, but is new to the Bitcoin P2P layer.

This definition is per the code comments in Bitcoin Core written by Pieter =
Wuille:

&lt;pre&gt;
Variable-length integers: bytes are a MSB base-128 encoding of the number.
The high bit in each byte signifies whether another digit follows. To make
the encoding is one-to-one, one is subtracted from all but the last digit.
Thus, the byte sequence a[] with length len, where all but the last byte
has bit 128 set, encodes the number:

  (a[len-1] &amp; 0x7F) + sum(i=3D1..len-1, 128^i*((a[len-i-1] &amp; 0x7F)+=
1))

Properties:
* Very small (0-127: 1 byte, 128-16511: 2 bytes, 16512-2113663: 3 bytes)
* Every integer has exactly one encoding
* Encoding does not depend on size of original integer type
* No redundancy: every (infinite) byte sequence corresponds to a list
  of encoded integers.

0:         [0x00]  256:        [0x81 0x00]
1:         [0x01]  16383:      [0xFE 0x7F]
127:       [0x7F]  16384:      [0xFF 0x00]
128:  [0x80 0x00]  16511: [0x80 0xFF 0x7F]
255:  [0x80 0x7F]  65535: [0x82 0xFD 0x7F]
2^32:           [0x8E 0xFE 0xFE 0xFF 0x00]
&lt;/pre&gt;

=3D=3D=3D=3D Checkpoints =3D=3D=3D=3D

A &#39;&#39;checkpoint&#39;&#39; is defined for a block as a tuple of its h=
ash and the chain work:

{| class=3D&quot;wikitable&quot;
! Field Name
! Data Type
! Byte Size
! Description
|-
| block_hash
| uint256
| 32
| The hash of the block
|-
| chain_work
| VarInt
| Variable(1-20)
| A delta between the total work in the chain at the checkpoint block and a=
 previous checkpoint, determined by context
|}

=3D=3D=3D Service Bit =3D=3D=3D

This BIP allocates a new service bit:

{| class=3D&quot;wikitable&quot;
|-
| NODE_HEADERS_V2
| &lt;code&gt;1 &lt;&lt; ?&lt;/code&gt;
| If enabled, the node MUST respond to &lt;code&gt;getcheckpts&lt;/code&gt;=
 and &lt;code&gt;getheaders2&lt;/code&gt; queries
|}

=3D=3D=3D New Messages =3D=3D=3D

=3D=3D=3D=3D getcheckpts =3D=3D=3D=3D
&lt;code&gt;getcheckpts&lt;/code&gt; is used to request block headers at a =
specified distance from each other which serve as checkpoints during parall=
el header download. The message contains the following fields:

{| class=3D&quot;wikitable&quot;
! Field Name
! Data Type
! Byte Size
! Description
|-
| block_locator
| uint256[]
| Variable
| A vector of block hashes in descending order by height used to identify t=
he header chain of the requesting node
|-
| interval
| uint32_t
| 4
| The distance in block height between requested block hashes
|}

# Nodes SHOULD NOT send &lt;code&gt;getcheckpts&lt;/code&gt; unless the pee=
r has set the &lt;code&gt;NODE_HEADERS_V2&lt;/code&gt; service bit
# The hashes in &lt;code&gt;block_locator&lt;/code&gt; MUST be in descendin=
g order by block height
# The block locator SHOULD be generated as it is in &lt;code&gt;getheaders&=
lt;/code&gt; requests
# The receiving node MUST respond to valid requests with a &lt;code&gt;chec=
kpts&lt;/code&gt; response where the interval is the same as in the request=
 and the first checkpoint hash matches the first common block hash in the b=
lock locator

=3D=3D=3D=3D checkpts =3D=3D=3D=3D
&lt;code&gt;checkpts&lt;/code&gt; is sent in response to &lt;code&gt;getche=
ckpts&lt;/code&gt;, listing block hashes at the specified interval. The mes=
sage contains the following fields:

{| class=3D&quot;wikitable&quot;
! Field Name
! Data Type
! Byte Size
! Description
|-
| start_height
| uint32_t
| 4
| The height of the first block in the active chain matching the request&#3=
9;s block locator
|-
| end_height
| uint32_t
| 4
| The height of the last block in the active chain
|-
| start_checkpoint
| Checkpoint
| 48
| The checkpoint structure for the block in the active chain at height star=
t_height
|-
| end_checkpoint
| Checkpoint
| 48
| The checkpoint structure for the block in the active chain at height end_=
height
|-
| interval
| uint32_t
| 4
| The distance in block height between checkpoints
|-
| checkpoints_length
| CompactSize
| Variable(1-5)
| The number of checkpoints to follow
|-
| checkpoints
| Checkpoint[]
| checkoints_length * Variable(33-52)
| The checkpoints as specified below
|}

# The interval SHOULD match the field in the &lt;code&gt;getcheckpts&lt;/co=
de&gt; request
# The start_checkpoint SHOULD correspond to the first block hash in the loc=
ator from the &lt;code&gt;getcheckpts&lt;/code&gt; request that is part of =
the active chain
# The end_checkpoint SHOULD correspond to the tip of the node&#39;s active =
chain
# The start_height MOST be set to the block height of the start_checkpoint
# The end_height MOST be set to the block height of the end_checkpoint
# If the interval is zero, the checkpoints vector MUST be empty
# If the interval is non-zero, checkpoints MUST correspond to blocks on the=
 active chain between the start_checkpoint and the end_checkpoint (exclusiv=
e), where the difference in block height between each entry and the previou=
s one is equal to the interval
# The checkpoints_length MUST be less than or equal to 2,000
# The node SHOULD include as many checkpoints on its active chain as are av=
ailable, up to the limit of 2,000
# The chain_work field in the first checkpoint MUST be the total work in th=
e chain ending at that block
# The chain_work field in each subsequent checkpoint MUST be the difference=
 in chain work between that block and the previous checkpoint
# The chain_work field in each checkpoint MUST be a properly-encoded VarInt=
, not exceeding 20 bytes

=3D=3D=3D=3D getheaders2 =3D=3D=3D=3D
&lt;code&gt;getheaders2&lt;/code&gt; is used to request compressed headers =
for a range of blocks. The message contains the following fields:

{| class=3D&quot;wikitable&quot;
! Field Name
! Data Type
! Byte Size
! Description
|-
| max_version
| uint8_t
| 1
| The maximum supported encoding version of the headers
|-
| flags
| uint8_t
| 1
| A bitfield of message encoding flags
|-
| start_height
| uint32_t
| 4
| The height of the first block header in the requested range
|-
| end_hash
| uint256
| 32
| The hash of the last block header in the requested range
|}

# Nodes SHOULD NOT send &lt;code&gt;getheaders2&lt;/code&gt; unless the pee=
r has set the &lt;code&gt;NODE_HEADERS_V2&lt;/code&gt; service bit
# The height of the block with hash end_hash MUST be greater than or equal =
to start_height, and the difference MUST be strictly less than 3,000
# The end_hash SHOULD match one in a previously received &lt;code&gt;checkp=
ts&lt;/code&gt; message, otherwise the receiving node MAY disconnect
# The 0th bit (least significant order) of the flags field MAY be set to re=
quest the coinbase transaction and merkle branch for the block at height st=
art_height

=3D=3D=3D=3D headers2 =3D=3D=3D=3D
&lt;code&gt;headers2&lt;/code&gt; is sent in response to &lt;code&gt;gethea=
ders2&lt;/code&gt;, listing the compressed headers in the requested range. =
The message contains the following fields:

{| class=3D&quot;wikitable&quot;
! Field Name
! Data Type
! Byte Size
! Description
|-
| version
| uint8_t
| 1
| The encoding version of the headers
|-
| flags
| uint8_t
| 1
| A bitfield of message encoding flags
|-
| start_height
| uint32_t
| 4
| The height of the first block header returned
|-
| headers_length
| CompactSize
| 1-3
| The number of block headers to follow
|-
| headers
| CompressedHeader[]
| Variable
| The compressed block headers
|-
| start_block_coinbase_tx
| CTransaction
| Variable
| The coinbase transaction in the block at start_height
|-
| start_block_coinbase_branch
| uint256[]
| Variable
| A merkle branch linking the coinbase transaction in the block at start_he=
ight to its header
|}

# The version MUST be less than or equal to the max_version field of the &l=
t;code&gt;getheaders2&lt;/code&gt; request
# Any bits set in the flags field of the &lt;code&gt;getheaders2&lt;/code&g=
t; request MAY be set in the response field
# Any bits not set in the flags field of the &lt;code&gt;getheaders2&lt;/co=
de&gt; request MUST NOT be set in the response field
# The first header MUST be encoded with a 0-byte indicator (ie. the header =
is uncompressed)
# start_height MUST be set to the block height of the first header
# The hash of the last block SHOULD equal the end_hash of the &lt;code&gt;g=
etheaders2&lt;/code&gt; request, &#39;&#39;even if the block is no longer p=
art of the active chain&#39;&#39;
# The length of the headers vector MUST be less than or equal to 3,000
# The headers MUST be sequential in order of height, with each header a suc=
cessor of the previous one
# Each header SHOULD be optimally compressed
# The start_block_coinbase_tx should be the serialized coinbase transaction=
 in the block corresponding to the first header
# The start_block_coinbase_branch should be a vector of right-hand-side has=
hes in the merkle branch linking the coinbase transaction to the first head=
er, in order from bottom of the tree to top
# If the 0th bit (least significant order) of the flags field is unset, the=
 start_block_coinbase_tx and start_block_coinbase_branch fields MUST be omi=
tted

=3D=3D=3D Sync Strategies =3D=3D=3D

The general header sync protocol for clients now is to first request checkp=
oints from all peers with &lt;code&gt;getcheckpts&lt;/code&gt;, then decide=
 which peers to fetch ranges of headers from and download them with &lt;cod=
e&gt;getheaders2&lt;/code&gt;.

=3D=3D=3D=3D Forward Sequential Syncing =3D=3D=3D=3D

Similar to the current sync protocol, a client may choose one peer to downl=
oad headers from, then fetch them in forward sequential order. Once this pe=
er is out of headers, the client performs the same routine with any peers o=
ffering more headers.

With this strategy, the client is able to fully validate the block headers =
in order and abort if the peer serves an invalid one. On the other hand, th=
e peer may be able to serve a longer, lower-work chain than the global acti=
ve chain, wasting the client&#39;s time, memory, and storage space.

=3D=3D=3D=3D Parallel Header Download =3D=3D=3D=3D

In order to increase the throughput of header downloads, a node may downloa=
d multiple header ranges in parallel from all peers serving the same checkp=
oints, then validate them in sequential order.

=3D=3D=3D=3D Random Sampling Proof-of-Work  =3D=3D=3D=3D

Similar the FlyClient&lt;ref&gt;<a href=3D"https://www.youtube.com/watch?ti=
me_continue=3D8400&amp;v=3DBPNs9EVxWrA" target=3D"_blank">https://www.<wbr>=
youtube.com/watch?time_<wbr>continue=3D8400&amp;v=3DBPNs9EVxWrA</a>&lt;/<wb=
r>ref&gt; header download protocol, clients can select the peer claiming th=
e greatest total work chain and use random sampling to efficiently determin=
e if the peer is likely to be reporting its chain work honestly.

The client treats the checkpoint message as a commitment to chain work of i=
ntermediate ranges of headers, the client then randomly samples ranges of h=
eaders weighted by total work to determine whether the total chain work is =
valid before downloading all headers. To defend against malicious peers att=
empting to reuse earlier headers later in the chain to fake greater total w=
ork, the client should check the block height in the coinbase transaction f=
or all headers after the BIP 34 activation height. If the peer is found to =
be dishonest, they can be banned before the client downloads too many heade=
rs, otherwise the client chooses this as the primary sync peer for forward =
sequential sync or parallel download.

=3D=3D Rationale =3D=3D

* &#39;&#39;&#39;Why include the coinbase transaction in the headers messag=
es?&#39;&#39;&#39; The primary reason is that after BIP 34&lt;ref&gt;<a hre=
f=3D"https://github.com/bitcoin/bips/blob/master/bip-0034.mediawiki" target=
=3D"_blank">https://github.com/<wbr>bitcoin/bips/blob/master/bip-<wbr>0034.=
mediawiki</a>&lt;/ref&gt; activation at block height 227,835, coinbase tran=
sactions constitute cryptographic commitments to a block&#39;s height in th=
e chain, which mitigates certain attacks during header sync. Furthermore, t=
he &lt;code&gt;getheaders2&lt;/code&gt; message can be used as a simple way=
 of requesting a coinbase transaction for a single header, which may be ind=
ependently useful.

* &#39;&#39;&#39;Why not omit nBits entirely?&#39;&#39;&#39; The compressio=
n is designed to permit full decompression of all headers in a &lt;code&gt;=
headers2&lt;/code&gt; message &#39;&#39;without&#39;&#39; requiring any oth=
er chain context. This is desirable so that proofs of work may be validated=
 for arbitrary header ranges. While nBits can be computed knowing previous =
headers, this requires block headers that may not be sent in the same messa=
ge.

=3D=3D Compatibility =3D=3D

This is backwards compatible, as it defines new P2P messages which are avai=
lable if a service bit is signaled. There are no changes to consensus rules=
.

=3D=3D Acknowledgements =3D=3D

Thanks to Gregory Maxwell for suggestions on the compressed header encoding=
 and the DOS-resistant sync strategies. Thanks to Suhas Daftuar for helpful=
 discussions.

Credit for the VarInt encoding goes to Pieter Wuille.</pre></div></div>
<br>______________________________<wbr>_________________<br>
bitcoin-dev mailing list<br>
<a href=3D"mailto:bitcoin-dev@lists.linuxfoundation.org">bitcoin-dev@lists.=
<wbr>linuxfoundation.org</a><br>
<a href=3D"https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev" =
rel=3D"noreferrer" target=3D"_blank">https://lists.linuxfoundation.<wbr>org=
/mailman/listinfo/bitcoin-<wbr>dev</a><br>
<br></blockquote></div><br><br clear=3D"all"><div><br></div>-- <br><div cla=
ss=3D"gmail_signature" data-smartmail=3D"gmail_signature"><div dir=3D"ltr">=
Riccardo Casatta - <a href=3D"https://twitter.com/RCasatta" target=3D"_blan=
k">@RCasatta</a></div></div>
</div></div></div>

--0000000000006aa04f056888c1c9--